1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle, and more particularly to a control system for a hybrid vehicle employing a series hybrid configuration in which an internal combustion engine is used solely for the purpose of driving a generator.
2. Background Art
Heretofore, there has been known a hybrid vehicle designed to be driven by a combination of a plurality of drive power sources adapted to be operated simultaneously or individually. A drive train for the hybrid vehicle includes one type called “series hybrid configuration”. In this series hybrid configuration, an internal combustion engine (hereinafter referred to simply as “engine”) is used solely for the purpose of driving a generator, and a direct drive of road wheels (i.e., driving road wheels) of the hybrid vehicle is performed only by a vehicle-driving motor. An electric energy generated by the generator is directly used for driving the vehicle-driving motor, or after being appropriately stored in a battery, extracted (discharged) from the battery according to need and supplied to the vehicle-driving motor.
For example, JP 2005-033886A discloses a control system for a hybrid vehicle employing a series hybrid configuration. In this system, a converter (a device for frequency/voltage conversion) and an inverter (a device for converting a direct current to an alternating current) are provided between a generator and a battery and between the battery and a vehicle-driving motor, respectively.
The frequency/voltage or DC-AC conversion using the converter or inverter or involves a problem about relatively large energy loss during the conversion. This energy loss will lead to deterioration in fuel economy.
As one of the measures to prevent such energy loss, a drive train may be designed to supply an AC energy generated by a generator, directly to a vehicle-driving motor (AC motor) without through a converter and an inverter. In this configuration, considering that the vehicle-driving motor for directly driving road wheels is essentially required to generate an output (i.e., vehicle power) complying with a driver's request (manipulation) including requests for vehicle acceleration and vehicle deceleration, an electric energy must be supplied to the vehicle-driving motor in conformity to the required vehicle power. Specifically, in the drive train designed to supply an AC energy generated by the generator, directly to the vehicle-driving motor, it is necessary to allow an output of the generator to have a current waveform (amplitude, frequency and phase) conforming to that required for the vehicle-driving motor.
Further, the required output of the vehicle-driving motor is frequently changed depending on vehicle running conditions, and the current waveform required for the vehicle-driving motor is changed accordingly. Therefore, the output (current waveform) of the generator has to be changed in response to the change in the required current waveform. In reality, the output of the generator is not always changed at a sufficiently-high response speed. For example, if a frequency of an alternating current required for the vehicle-driving motor is rapidly changed, a frequency of an output alternating current of the generator has to be responsively changed at a high speed. However, the frequency of the output alternating current of the generator is proportional to an engine speed, and thereby it is impossible to change the frequency at a higher speed than a change rate of the engine speed. Thus, if the engine speed is changed with a certain delay, a waveform mismatch (waveform difference) will occur in proportion to the delay.
The waveform difference is likely to cause difficulty in obtaining a required vehicle power (vehicle driving force), which leads to negative effects, such as torque shock. Therefore, there is the need for, in response to occurrence of such a waveform difference, eliminating the waveform difference as quickly as possible.